JERUSALEM—Iran has sent Syria a sophisticated radar system that could threaten Israel's ability to launch a surprise attack against Iran's nuclear facilities, say Israeli and U.S. officials, extending an alliance aimed at undermining Israel's military dominance in the region.

The radar could bolster Syria's defenses by providing early warning of Israeli air-force sorties. It could also benefit Hezbollah, the Iran-backed militant group based in Lebanon and widely believed to receive arms from Syria.

Any sharing of radar information by Syria could increase the accuracy of Hezbollah's own missiles and bolster its air defenses. That would boost Hezbollah defenses, which U.S. and Israeli officials say have been substantially upgraded since 2006, the last time Israel fought the southern Lebanon-based group.

The mid-2009 transfer was described in recent months by two Israeli officials, two U.S. officials and a Western intelligence source, and confirmed Wednesday by the Israeli military. Though they didn't name the system's final recipient in Syria, these and other officials described it as part as a dramatic increase in weapons transfers and military coordination among Iran, Syria and Hezbollah.

Iran and Syria both denied that a radar transfer took place.

The increased sophistication of the weapons transfers and military cooperation among the three signal an increased risk of conflict on Israel's northern border. U.S. officials worry any new fighting would be more likely to include Syria, which hasn't directly engaged Israeli in combat since 1974.

The radar transfer could potentially violate a 2007 United Nations Security Council resolution that bans Iran from supplying, selling or transferring "any arms or related material."

Though officials say the transaction took place about a year ago, Israel and the U.S. haven't publicized it, a departure from years past when Israeli officials were often eager to trumpet Iranian arms transfers to Syria and Hezbollah as violations of Security Council resolutions.

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